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16 April 2008 Efficiency and lifetime enhancement of phosphorescent organic devices
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A way to reach highly efficient and stable red bottom emission organic light emitting diodes (OLEDs) is the use of doped transport layers, charge and exciton blockers, and phosphorescent emitter materials to combine low operating voltage and high quantum yield. We will show how efficiency and lifetime of such devices can be further increased. In our contribution, we report on highly efficient red p-i-n type organic light emitting diodes using an iridium-based electrophosphorescent dye, Ir(MDQ)2(acac), doped in α-NPD as host material. By proper adjustment of the hole blocking layer, the device performance may be enhanced to 20 % external quantum efficiency at an operation voltage of 2.4 V and a brightness of 100 cd/m2. At the same time, a power efficiency of 37.5 lm/W is reached. The quantum efficiency is well above previously reported values for this emitter. We attribute this high efficiency to a combination of a well-adjusted charge carrier balance in the emission layer and a low current density needed to reach a certain luminance due to the use of doped transport layers. High chemical stability of the blocker material assures a long device lifetime of 32.000 hours at 1.000 cd/m2 initial luminance.
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Rico Meerheim, Sebastian Scholz, Gregor Schwartz, Sebastian Reineke, Selina Olthof, Karsten Walzer, and Karl Leo "Efficiency and lifetime enhancement of phosphorescent organic devices", Proc. SPIE 6999, Organic Optoelectronics and Photonics III, 699917 (16 April 2008);

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